The present invention relates to a fuel system for delivering fuel under high pressure to a cylinder of an associated compression ignition internal combustion engine. In particular, but not exclusively, the invention relates to a fuel pump arranged to deliver fuel to an injector for delivering fuel to a cylinder or other combustion space of the associated engine.
In a known fuel system for an internal combustion engine, a fuel pump for pressurising fuel to high pressure is arranged to deliver fuel through a high-pressure fuel line to a fuel injector. The fuel pump may take the form of a unit pump in which a spill valve for controlling communication between a pump chamber and a low pressure drain is integrally formed within a main pump housing. In addition, or alternatively, the injector may be integrally formed with the fuel pump in a unit pump/injector arrangement. The injector typically includes a valve needle which is engageable with a valve seating to control the injection of fuel into the engine. In use, movement of the valve needle away from its seating permits fuel to flow from an injector delivery chamber, through an injector outlet opening, into a cylinder of the associated engine.
In such fuel systems, it is thought to be desirable to limit the pressure of fuel within the system in order to protect the component parts of the pump and/or the injector. In the event that a fault occurs in the fuel system, or in the event of system failure, exposure of the components to excessively high fuel pressures can result in irreparable damage to the component parts. Additionally, in order to maximize fuel pressures under normal engine operating conditions, it may be desirable to choose system parameters which produce fuel pressures in excess of a safe level at extreme engine running conditions. In such circumstances, it is also desirable for the pressure of fuel within the system to be limited.
According to a first aspect of the present invention, there is provided a fuel system which is arranged to deliver fuel at high pressure to a fuel injector through a high pressure fuel line, the fuel system including a pressure relief valve arrangement for relieving fuel pressure within the high pressure fuel line, wherein the pressure relief valve arrangement includes:
a first housing part defining a first valve surface and a second housing part defining a second valve surface, wherein the high pressure fuel line is defined, at least in part, within the first and second housing parts and wherein the first valve surface is engageable with but separable from the second valve surface, clamping means for applying a clamping load to at least one of the first and second housing parts so as to urge the first and second valve surfaces into substantially sealing engagement, wherein the clamping means is arranged to permit separation of the first and second valve surfaces, against the clamping load, in the event that fuel pressure within the high pressure fuel line exceeds a predetermined amount, thereby to permit fuel pressure within the high pressure fuel line to be relieved.
In the event that fuel pressure within the high pressure line exceeds a predetermined maximum level, forces due to high fuel pressure will tend to separate the first and/or second housing parts and are sufficient to overcome the clamping load applied by the clamping means, thereby permitting a degree of separation between the first and second housing parts. Under such circumstances, fuel within the high-pressure fuel line is able to leak between the first and second valve surfaces to a low-pressure drain or reservoir, thereby causing fuel pressure within the high pressure fuel line to be reduced. In this way, any increase in fuel pressure within the fuel system, which may otherwise cause damage to the component parts of the system, can be avoided.
The fuel system may take the form of a fuel pump for delivering fuel to the injector. For example, the fuel system may take the form of a unit fuel pump.
The fuel system may also include the injector. For example, the fuel system may include a fuel pump and a fuel injector which are integrally formed in a unit pump/injector arrangement. It will be appreciated, however, that the fuel system need not include the injector.
In a preferred embodiment, at least one of the first and second valve surfaces is provided with a relief which is sealingly engageable with the other of the first and second surfaces upon application of the clamping load, the relief being separable from said other of the first and second valve surfaces in the event that fuel pressure within the high pressure fuel line exceeds the predetermined amount. Preferably, the relief on the surface may take the form of a projection of annular form or may comprise two or more annular projections.
The relief is preferably shaped to define a first area which is exposed to fuel pressure within the high pressure fuel line, the first area being selected to ensure separation of the first and second valve surfaces occurs at the required, maximum fuel pressure for a given clamping load.
In one embodiment of the invention, the first valve surface is provide with a first relief and the second valve surface is provided with a second relief, the first and second reliefs being shaped to sealingly engage one another upon application of the clamping load and being separable in the event that fuel pressure within the high pressure fuel line exceeds the predetermined amount.
This arrangement is particularly advantageous if it is necessary for drillings, passages or other components to be accommodated within the first and/or second housing parts.
In a preferred embodiment, the relief provided on the first or second valve surface defines, together with the other of the first and second valve surfaces, a restricted clearance through which fuel flows from the high pressure fuel line to a region of low pressure at a restricted rate when the first and second valve surfaces are caused to separate.
Preferably, the clamping load is applied by means of a cap nut. The cap nut preferably forms at least a part of the clamping means for applying the clamping load to the second housing part. Conveniently, the second housing part may take the form of an adapter plate forming part of the fuel pump.
According to a further aspect of the present invention, there is provided a pressure relief valve arrangement for use in a fuel system as herein described, comprising first and second housing parts which define, at least in part, a high pressure fuel line, first and second valve surfaces being defined by the first and second housing parts respectively and being shaped for sealing engagement with one another upon application of the clamping load by the clamping means, and being separable from one another in the event that fuel pressure within the high pressure fuel line exceeds the predetermined amount.
According to a still further aspect of the invention, there is provided a fuel pump arranged to deliver fuel at high pressure to a fuel injector through a high pressure fuel line, the fuel pump comprising a pressure relief valve arrangement for relieving fuel pressure within the high pressure fuel line, including:
a first housing part defining a first valve surface and a second housing part defining a second valve surface, wherein the high pressure fuel line is defined, at least in part, within the first and second housing parts and wherein the first valve surface is engageable with but separable from the second valve surface,
clamping means for applying a clamping load to the first and second housing parts so as to urge the first and second valve surfaces into substantially sealing engagement, wherein the clamping means is arranged to permit separation of the first and second valve surfaces, against the clamping load, in the event that fuel pressure within the high pressure fuel line exceeds a predetermined amount, thereby to permit fuel within the high pressure fuel line to flow to a region of relatively low pressure to relieve fuel pressure within the high pressure fuel line.